EP0058186B1 - Photographic enlarger with pinpoint light source - Google Patents

Description

The present invention relates to a photographic enlarger making it possible to enlarge a negative or a slide to the desired formats and densities by producing a projection of these on a sheet of photosensitive paper, this enlarger comprising a point light source, an enlargement objective , an optical system creating an image of the light source at the optical center of the enlargement objective and a mobile negative holder, the point light source moving axially according to a linear law as a function of the axial displacement of the objective enlargement.

In the photographic laboratory, the enlarger makes it possible to enlarge to the desired formats and densities, a negative or a slide by carrying out a projection of these on a sheet of photosensitive paper knowing that the optical system with condensers must form an image of the lamp in the optical center of the enlargement lens. It is also necessary that the light cone from the optical system completely covers the negative. It should be noted that the axial image-object displacement in this simple optical system responds to the equation of the form: Y = 1 / X, which represents a hyperbolic function. Therefore the axial displacement of the enlargement lens generates a corresponding displacement of the source, the two displacements being linked by the above function. In practice, this means that adjustment cannot be carried out mechanically in a simple manner and requires complicated manual intervention.

On photographic enlargers of this type, the image of the lamp by the optical system with condensers having to be done in the optical center of the lens, the adjustment of the lamp must be carried out after each change of position of the lens, that is to say after each adjustment, setting which is tedious, long and source of errors.

On the other hand, the image of the point light source in the objective being also point, all the light energy passes through the center of the lenses and of the diaphragm of the objective. We therefore see that the diaphragm loses its function, becoming useless at dimming the illumination, this is how in the case of small enlargement ratios where there is a strong non-attenuable illuminance, we are forced to considerably reduce the exposure times to obtain the appropriate lumination for the sensitive surface to be exposed since H = E - t (where H is the lumination, E the illumination and t the exposure time), but this relation not being true that in a very limited domain (Schwartzchild's law) this results in poor quality of the final image. The cone of light coming from the condensers having to completely cover the negative while having a minimum safety margin so as to illuminate it as much as possible, requires the adjustment or the change of the optical system with condensers to adapt to the format of the negative. This system is a source of errors, it is very expensive in the case of changing condensers, it is tedious to use and therefore causes a considerable loss of time.

All these drawbacks mean that this type of enlarger is no longer used, except in very specific cases where the given contrast and definition are imperative (in the graphic arts for example).

It is for this reason that modular enlargers are more commonly used. They have a frame on which you can adapt a point light system, semi-diffuse or diffuse, or color (exclusively diffuse) by a set of interchangeable boxes. Often, it is necessary to have optical systems relating to each box and corresponding to each negative format encountered. As a result, these devices are expensive and tedious to use. It should also be noted that the point light source is very easily diffusable (to meet artistic needs, in the drawing of portraits for example) while the converse is impossible. It is indeed impossible to transform a diffuse source into a point source.

A principle similar to that of the enlarger according to the invention is used in the projector described by US-A-2 843 010. This spot light projector, has a single optical group, in which the source is connected to the objective by a linear displacement system. The law of displacement of the source and the objective is not linear, since this displacement answers a hyperbolic function. Within a certain operating range the approximation can be considered satisfactory. This approximation can be considered acceptable for large objectives, for a relatively limited displacement.

The object of the present invention is to remedy the aforementioned drawbacks of enlargers with a point light source. For this purpose, the enlarger according to the invention is characterized in that the optical system consists of two fixed optical systems A and B, such that the image focus of system A is merged with the focus object of system B.

According to a preferred embodiment, the two optical systems have identical focal lengths. In this case, the same mobile carriage can carry the enlargement objective and the light source so that the relative axial displacement of the objective and the source is zero. This adjustment can therefore be carried out mechanically, which is not the case when the law of relative displacements is hyperbolic, as was the case in the point source enlargers of the prior art.

In addition, the enlarger preferably has a negative holder at different levels, which eliminates changes or adjustments to the condenser optical system.

The advantages obtained thanks to this system are on the one hand the elimination of tedious adjustments, on the other hand the reduction in the cost of the enlarged because firstly, the light being punctual, it is always possible to diffuse it without needing any other light boxes and secondly by its negative negative holder which does not impose several optical systems adapted to the formats encountered. It also has the advantage of being brighter during large enlargement ratios. In short, this enlarger takes up the principle of the punctual light enlarger while keeping all its qualities, the optical system eliminating all its defects. The whole optical system and negative holder makes it a very flexible device to use and always adapting to the artistic needs of the operator while remaining very fast, pleasant and simple to use.

• la figure 1 représente schématiquement le système optique de l'agrandisseur selon l'invention,
• les figures 2 à 4 représentent les fonctions du déplacement objet pA en ordonnée par rapport au déplacement image pB en abscisse, la fig. 2 dans la technique antérieure, la fig. 3 pour le système optique selon l'invention lorsque fA = fB, la fig. 4 lorsque fA :0 fB,
• les figures 5 à 7 illustrent la technique antérieure lors du changement d'un négatif N3 par un négatif N2 de format inférieur, où l'on change le système optique (fig. 7, fig. 6) et où l'on change la position du système optique (fig. 7, fig. 5) pour l'adapter au format du négatif N2.

The present invention will be better understood with reference to the description of an exemplary embodiment and the attached drawing in which:

• FIG. 1 schematically represents the optical system of the enlarger according to the invention,
• FIGS. 2 to 4 represent the functions of the object displacement pA on the ordinate with respect to the image displacement pB on the abscissa, FIG. 2 in the prior art, FIG. 3 for the optical system according to the invention when fA = fB, FIG. 4 when fA: 0 fB,
• Figures 5 to 7 illustrate the prior art when changing a negative N3 by a negative N2 of smaller format, where we change the optical system (fig. 7, fig. 6) and where we change the position of the optical system (fig. 7, fig. 5) to adapt it to the format of the negative N2.

Referring to fig. 1, we see the optical diagram of the enlarger according to the invention comprising a lamp 1 which constitutes the point light source Ja moving between A and F _A along the optical axis represented by a mixed line, two fixed optical groups A and B , each being composed of a lens or a group of lenses (group A can also be constituted by a spherical mirror), these optical groups of focal length fA and fB have their focal point F ',, (image of A,) and F _a (object of B) combined and their displacement function is represented by FIGS. 3 and 4. The light cone from the last optical group B meets a series of segments N1, N2 and N3 representing negatives of different formats used in this enlarger and whose position depends on the size. The image 2 of the light source 1 by the optical system AB must form, as the line shows, at the optical center of the enlargement objective O translating along the optical axis by the focusing system point.

où x est la taille de l'objet et y la taille de l'image.Fig. 1 represents the optical system of an enlarger adjusted in such a way that the image 2 of the lamp 1 formed by the optical system is made in the optical center of the objective placed at O and this whatever the position of the lens without having to manually adjust. The mathematical approach gives us the object size / image size ratio, that is to say in Figure 1:

where x is the size of the object and y is the size of the image.

The size of the object therefore gives a size image proportional to the focal ratio and is therefore constant. It is thus possible to use a light source whose size will be correctly calculated so that its image in the optical center corresponds to the size of the open diaphragm. The usefulness of the diaphragm is thus preserved without losing the relative punctuality of the light source.

où pB représente le déplacement image, où pA représente le déplacement objet, et où z représente la distance d'un rayon lumineux issu de l'objet à l'axe optique. Cette équation est de la forme Y = AX + B qui est l'équation d'une droite. De plus lorsque fA = fB l'équation se résout à pB = pA. Le déplacement relatif entre la lampe 1 et son image 2 (c'est-à-dire l'objectif) est donc ou proportionnel ou nul. Il est donc facile d'automatiser le déplacement en déplaçant un chariot supportant la lampe soit directement par le chariot supportant l'objectif (système de mise au point) lorsque le déplacement est nul, soit, lorsque le déplacement est proportionnel, au moyen de deux câbles s'enroulant sur deux tambours d'un même treuil dont le rapport des diamètres correspond au rapport

Knowing that:

where pB represents the image displacement, where pA represents the object displacement, and where z represents the distance of a light ray coming from the object to the optical axis. This equation is of the form Y = AX + B which is the equation of a straight line. Furthermore when fA = fB the equation is solved at pB = pA. The relative displacement between the lamp 1 and its image 2 (that is to say the objective) is therefore either proportional or zero. It is therefore easy to automate the movement by moving a carriage supporting the lamp either directly by the carriage supporting the lens (focusing system) when the movement is zero, or, when the movement is proportional, by means of two cables winding on two drums of the same winch whose diameter ratio corresponds to the ratio

In FIG. 1 we also see the different locations of the negatives indicated by the segments N1, N2, N3. These negatives are placed so that they are completely covered by the light cone from the optical group B. A film holder allowing the corresponding negative to be maintained can be placed on different levels using slides or using an adjustment system. This system has the effect of avoiding any change or adjustment of the optical system of the enlarger. Since the rest of the enlarger is constructed identically to the prior art, a more detailed description is unnecessary.

It therefore follows that the enlarger according to the invention is an enlarger with a point light source requiring no adjustment of the lamp during changes of focus, requiring no adjustment or change of optics during changes of negative format and whose usefulness of the diaphragm is preserved.

Claims (6)

1. Photographic enlarger enabling the enlargement of a negative or transparency to desired formats and densities by creating a projection of the former on a sheet of photosensitive paper, this enlarger having a pinpoint light source, an enlarging lense, an optical system creating an image of the light source at the optical centre of the enlarging lens and a mobile negative holder support, the pinpoint light source being displaced axially in accordance with a linear law as a function of the axial displacement of the enlarging lens, characterised in that the optical system is formed of two fixed optical systems A and B so that the focal image of system A is merged with the focal object of system B.

2. Enlarger according to Claim 1, characterised in that the two optical systems have identical focal distances.

3. Enlarger according to Claim 1, characterised in that the two optical systems A and B have different focal distances fA and fB.

4. Enlarger according to Claim 2, characterised in that the enlarging lens and the pinpoint light source are mounted on a single mobile trolley so that the relative displacement between this lens and this source is zero.

5. Enlarger according to Claim 3, characterised in that the enlarging lens and the pinpoint light source are mounted on two mobile trolleys simultaneously, these trolleys being arranged so that they move so that the ratio of their movements are proportional to (fB/_fA)².

6. Enlarger according to Claim 1, characterised in that the negative holder support has different levels corresponding to different formats of the negative.

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